Tonks shut her eyes when Charlie spoke a very real truth: She was supposed to be at home. She wasn't supposed to have ever been at the castle. She'd only had Teddy weeks ago, and she'd still been healing. There had been no denying that. No on in their right mind would have told her to be at the battle. But she had gone anyway. Because how could she not? When there were so many people she cared about who had been hurt by the war, and so many more who were at the castle that night? How could she have left Remus to fight by himself? How could she not go and try and keep him safe? It had all seemed rational when she had left home. She and Remus had survived this far; who would have thought their luck would run out then?
But she would have to carry that knowledge with her for as long as she was here. She was the reason her son was now an orphan. She hadn't saved Remus and she hadn't made it home herself. (Belatedly, she realized that Harry had never said which one of them died first; did one of them have to watch the other? The mere thought of it made her sick. The idea of watching Remus be killed was too much to bear. But then, she couldn't wish the opposite on him either.)
Either way, she wouldn't see her son again, and that knowledge tore at her more than she could say.
"I know," was the only thing Tonks managed to say.